To be generally acceptable, the foods, which definition here will include food ingredients and edible oils, need not only be nutritious, versatile and economical, but also be attractive in color, aroma, taste and texture. Plant protein preparations mainly from legumes and nuts, like soybeans, have unacceptable off-flavors which score heavily against their positive properties and limit their use.
Lipoxygenase enzyme has been recognized as the major cause of off-flavor in most vegetable protein sources including soybeans, peas and peanuts. Polyunsaturated fatty acids are oxidized through the catalytic action of lipoxygenase to initially produce hydroperoxides and finally yielding off-flavor causing agents like aldehydes, ketones, alcohols, furans, alpha-ketols and hydroxyacids.
Lipoxygenase is distributed through the cotyledons in legumes and many other seeds, but the enzyme is inactive because of its limited contact with oxygen and the substrate which are immobile due to the plant cell structure. Breaking of cell structure during the size reduction operation causes the oxidation to proceed. The control of off-flavors, therefore, requires inactivation of lipoxygenase enzyme.
Since lipoxygenase is heat sensitive, its inactivation is most commonly accomplished by thermal processing of the seeds. At temperatures above 60.degree. C. the half-lives of the various lipoxygenase enzymes rapidly decrease with increasing temperature. However, heat treatment also reduces the nitrogen solubility index and protein dispersibility index. Similarly, other currently available methods for the effective elimination or reduction of off-flavors in plant proteins are not very compatible with the preparation of their highly functional forms because they require conditions that cause denaturing of proteins. In addition, the removal of off-flavors in edible oils not only makes the oil refining process complex but also causes oil degradation. A method is needed to solve these problems in a natural and simple way.